I looked it up as I didn't know these rules either. From ITF rules of tennis 2010:
In doubles, the team due to serve in the first game of each set shall decide which player shall serve for that game. Similarly, before the second game starts, their opponents shall decide which player shall serve for that game. The partner of the player who served in the first game shall serve in the third game and the partner of the player who served in the second game shall serve in the fourth game. This rotation shall continue until the end of the set.
Well done by Grage/Zheng so far. I hope they win!